Acute Endothelin-1 Application Induces Reversible Fast Axonal Transport Blockade in Adult Rat Optic Nerve

Abstract

To investigate the effects of endothelin (ET)-1 on fast axonal transport in the optic nerve. Sterile sponge soaked in 1 nM ET-1 was applied to the optic nerve surface of adult Brown Norway rats for 1 hour, after which 20% horseradish peroxidase (HRP) was placed over the superior colliculi (SC). Rats were killed 2, 4, 6, 24, and 48 hours later; the retinas and optic nerves were removed and fixed; and cut sections were processed histochemically, to visualize the time course of HRP transport by light microscopy. Naive and saline controls were processed identically. Retinal ganglion cell (RGC) survival after acute ET-1 application was investigated in another group of animals. After retrograde labeling of RGCs with the fluorescent neurotracer Fluorochrome (FG; Fluorogold; Fluorochrome Inc., Denver, CO), 1 nM ET-1 was applied to the optic nerve. Rats were then killed 5, 10, and 21 days later. The retinas were whole mounted and FG-positive RGCs were imaged and quantified with fluorescence microscopy. In naive controls, HRP labeling was observed over the entire nerve at 2 hours but had cleared by 48 hours. HRP labeling of RGCs started at 6 hours, and by 48 hours, uniform labeling was seen throughout the retina. In ET-1-treated optic nerves, transport of HRP was arrested at the distal portion of the nerve at 2 hours. Recovery of transport was evident from 4 hours. At 6 and 24 hours, all nerves showed full recovery with HRP-positive RGCs in the retina, but the ratio of the RGC counts in treated versus fellow untreated eyes (0.66 +/- 0.13 and 0.67 +/- 0.17, respectively) was less than that of the naive control (1.02 +/- 0.28 and 1.05 +/- 0.13, respectively) animals. At 48 hours, recovery was complete, and there was no significant difference in the ratio of RGC counts between ET-1 and naive control groups. No RGC loss was observed after ET-1 application. Local acute application of ET-1 produces a reversible blockade of rapid axonal transport in optic nerve.